This invention relates to coplanar waveguides formed within electrically conductive sheets disposed on opposite surfaces of a dielectric substrate and, more particularly, to a hybrid coupler of electromagnetic power between the waveguides.
Circuit boards comprising a dielectric substrate with opposed surfaces covered by metallic electrically-conductive sheets are often used for construction of waveguides for conducting electromagnetic power among electronic components, such as radiators of an antenna, filters, phase shifters, and other signal processing elements.
There are three forms of such circuit boards. One form, known as strip-line, comprises a laminated structure of three electrically conductive sheets spaced apart by two dielectric substrates. The middle sheet is etched to form strip conductors which cooperate with the outer sheets, which serve as ground planes, to transmit a TEM (transverse electromagnetic) wave. A second form of the circuit board, known as microstrip, is also provided as a laminated structure, but is simpler than the strip-line in that there are only two sheets of electrically conductive material, the two sheets being spaced apart by a single dielectric substrate. One of the sheets is etched to provide strip conductors which in cooperation with the other sheet, which serves as a ground plane, supports a TEM wave. The third form of circuit board is provided with a coplanar waveguide, and comprises two sheets of electrically conductive material spaced apart by a dielectric substrate. The coplanar waveguide is formed completely within one of the sheets and is constructed as a pair of parallel slots etched within a conductive sheet, the two slots defining a central strip conductor. The central strip conductor cooperates with outer edges of the slot to support a TEM wave.
The coplanar waveguide structure is of particular interest herein because of its utility in interconnecting microwave components by use of a circuit board, which may be employed to support these components. Also, a TEM wave can be transmitted via a coplanar waveguide independently of the presence or absence of a conductive sheet on the opposite side of the circuit board. This permits greater flexibility in the layout of the circuit board since electrical components can be mounted on both sides of the board.
In the use of the circuit boards, it is frequently necessary to couple a portion of the power from one waveguide to another waveguide for combining signals such as, for example, in the construction of a Butler matrix for distributing electromagnetic signals among elements of a phased array antenna. The capability for coupling electromagnetic signals between waveguides provides for greater flexibility in the layout of components on the circuit board. This is particularly true in situations wherein power is to be coupled through the board between a waveguide on one side to a waveguide on the opposite side of the board. Heretofore, such coupling has been accomplished by use of a feedthrough connector with appropriate impedance matching structures.
A problem arises in the use of feedthrough connectors in combination with coplanar waveguides in that additional manufacturing steps are required. For example, a coplanar waveguide can be manufactured by photolithography including an etching of the pair of parallel slots which define the central strip conductor. In order to provide the feedthrough connector, it is necessary to drill a hole through the dielectric substrate, and then to establish an electrically conducting path through the drilled hole. Various techniques are available for establishing the electrically conducting path, including plating as well as the insertion of a metallic post. The drilling of holes and insertion of posts are totally separate manufacturing processes from those employed in the photolithography for construction of the coplanar waveguide. In addition, such feedthrough connector may also require additional impedance-matching structures to avoid unwanted reflections from a discontinuity in the waveguide presented by the feedthrough connector.